A recently developed type of secondary or rechargeable electrical conversion device comprises: (1) an anodic reaction zone containing a molten alkali metal anode-reactant, e.g., sodium, in electrical contact with an external circuit; (2) a cathodic reaction zone containing (a) a cathodic reactant comprising a liquid electrolyte, e.g., sulfur or a mixture of sulfur and molten polysulfide, which is electrochemically reversibly reactive with said anodic reactant, and (b) a conductive electrode which is at least partially immersed in said cathodic reactant; and (3) a solid electrolyte comprising a cation-permeable barrier to mass liquid transfer interposed between and in contact with said anodic and cathodic reaction zones. As used herein the term "reactant" is intended to mean both reactants and reaction products.
During the discharge cycle of such a device, molten alkali metal atoms such as sodium surrender an electron to an external circuit and the resulting cation passes through the solid electrolyte barrier and into the liquid electrolyte to unite with polysulfide ions. The polysulfide ions are formed by charge transfer on the surface of the porous electrode by reaction of the cathodic reactant with electrons conducted through the porous electrode from the external circuit. Because the ionic conductivity of the liquid electrolyte is less than the electronic conductivity of the porous electrode material, it is desirable during discharge that both electrons and sulfur be applied to and distributed along the surface of the porous conductive material in the vicinity of the cation-permeable solid electrolyte. When the sulfur and electrons are so supplied, polysulfide ions can be formed near the solid electrolyte and the alkali metal cations can pass out of the solid electrolyte into the liquid electrolyte and combine to form alkali metal polysulfide near the solid electrolyte.
During the charge cycle of such a device when a negative potential larger than the open circuit cell voltage is applied to the anode the opposite process occurs. Thus electrons are removed from the alkali metal polysulfide by charge transfer at the surface of the porous electrode and are conducted through the electrode material to the external circuit, and the alkali metal cation is conducted through the liquid electrolyte and solid electrolyte to the anode where it accepts an electron from the external circuit. Because of the aforementioned relative conductivities of the ionic and electronic phases, this charging process occurs preferentially in the vicinity of the solid electrolyte and leaves behind molten elemental sulfur. As can be readily appreciated the production of large amounts of sulfur near the surface of the cation-permeable membrane has a limiting effect on rechargeability. This is the case since sulfur is nonconductive and when it covers surfaces of the porous electrode, charge transfer is inhibited and the charging process is greatly hindered or terminated. Thus, in order to improve the rechargeability of a cell of this type it is necessary not only to supply polysulfide to the surface of the porous electrode in the vicinity of the cation-permeable membrane, but also to remove sulfur therefrom.
U.S. Pat. No. 3,811,493 and U.S. Pat. application Ser. No. 545,048 filed Jan. 29, 1975 both disclose energy conversion device designs which allow or promote improved mass transportation of reactants and reaction products to and from the vicinity of the solid electrolyte and the porous electrode during both discharge and charge. In the device disclosed in the patent an ionically conductive solid electrolyte is located between a first reactant in one container and a second reactant in another container. An electrode for one of the reactants comprises a layer of porous, electronically conductive material having one surface in contact with one side of the ionically conductive solid electrolyte and the other surface in contact with a structurally integral electronically conductive member permeable to mass flow of its reactant and electrically connected to the external circuit. An open volume exists between the structurally integral conductive member and the container wall to promote free flow and mixing of the reactant. Reactants also flow readily through the conductive member into the layer of porous electronically conductive material. The conductive member distributes electrons to the porous, conductive material which in turn transfers electrons to or from the reactants.
The improvement disclosed in the patent application comprises designing the cathodic reaction zone of the device such that there are a plurality of channels and/or spaces within said zone which are free of porous conductive electrodes and which are thus adapted to allow free flow of the molten cathodic reactants during operation of the device. This flow results from free convection within the channels and/or spaces, and from wicking of cathodic reactants within the conductive porous material.
U.S. Pat. application Ser. No. 567,464 filed Apr. 14, 1975 discloses an improved method for recharging secondary batteries or cells of the above-described type. The process involves maintaining a temperature gradient within the cathodic reaction zone during recharging such that the temperature of the cathodic reactants in a first region adjacent the solid electrolyte or cation-permeable barrier is sufficiently higher than the temperature of said reactants in a second region not adjacent the barrier such that sulfur in the first region vaporizes and is transported to said second region where it condenses.
The prior art designs disclosed and claimed in the aforementiond U.S. Patent and in Ser. No. 545,048 are effective in promoting distribution of reactants during both discharge and charge. However, even with these improved designs it is difficult to recharge the cells or batteries at high rates. The process of Ser. No. 567,464 overcomes recharging problems of the devices discussed above, but requires that heating means be employed to provide the necessary temperature gradients.
The improved device of this invention, like the process of Ser. No. 567,464 increases the ampere-hour capacity of the batteries or cells by promoting vapor transfer of sulfur within the cathodic reaction zone during recharging. Unlike the prior art process, however, the recharging device requires no temperature gradient and, thus, no additional external heating elements. Also, unlike the prior art process, the improved device of the invention is designed to provide specific channels for vapor transfer of sulfur within the cathodic reaction zone.